This invention concerns the routing of calls in a mobile telecommunication system.
Network elements relating to a known mobile telecommunications network and the connections between the network elements are presented in FIG. 1. The network used as an example in this application is according to the GSM standard. In the context of this application, the word network corresponds to a mobile telecommunication network operated by one operator. The solid lines in the figure illustrate connections comprising both signaling and traffic connections. The dashed lines correspond to pure signaling connections.
The network comprises Mobile Stations MS, Base Transceiver Stations (BTS1-BTS6), Base Station Controllers (BSC1, BSC2) and Mobile Switching Stations (MSCA, MSCB). The mobile switching stations are connected to their corresponding Visitor Locations Registers VLR. Additionally, the network comprises Home Location Registers HLR, and a Network Management System NMS.
The mobile stations MS are capable of establishing connections with the base stations BTS. A plurality of base transceiver stations are controlled by a base station controller BSC. Correspondingly, a plurality of base station controllers are controlled by a mobile switching center MSC. As an example, the connections established by the mobile station MS shown in the figure are routed from the base station BTS4 via the base station controller BSC2 to the mobile switching center MSCA. Depending on the location of the called subscriber, MSCA switches the calls back to base station controllers BSC1 or BSC2 controlled by MSCA, to other mobile switching centers such as MSCB or to other telecommunication networks such as the PSTN (Public Switched Telephone Network) or the ISDN (Integrated services Digital Network).
One feature characteristic of mobile telecommunication networks is that the geographical location of the mobile stations can change. Due to this, the network has to maintain information about the location of the mobile subscribers. In the network shown in FIG. 1, this information is distributed between the home location register HLR and the visitor location registers VLR being directly connected to mobile switching centers MSC. The HLR is in the home network of the mobile subscriber and contains the unchanging subscriber information and the location information of the mobile subscriber with an accuracy of one VLR area. The VLR area typically equals the area served by one mobile switching center. The VLR of the visited mobile switching center VMSC responsible for the area the subscriber is visiting at the time contains more exact information of the subscriber""s location.
The network management system NMS is used to collect information concerning the traffic situation of the network, for example, and to load configuration information and programs to other network elements.
The routing of a call to mobile subscriber B is shown in FIG. 2. In the figure, the network PLMNA (PLMN=Public Land Mobile Network) of the calling subscriber A, the home network HPLMNB of the called subscriber B, and the network VPLMNB visited by the called subscriber are shown. With PLMNA, HPLMNB and VPLMNB
all can be operated by the same operator (interoperator call),
two of them can be operated by the same operator and the third one by another operator, or
all can be operated by different operators.
In FIG. 2, the calling subscriber A is also a mobile subscriber. If the calling subscriber is a fixed network subscriber, the routing is almost completely alike, except that the gateway MSC GMSCA of subscriber A is replaced by a fixed network gateway exchange.
The signaling needed for establishing the call is presented in FIG. 3. The solid lines in the figure illustrate circuit switched connections comprising both signaling and user data traffic. The dashed lines correspond to packet switched signaling connections. First, a connection between the calling subscriber A and his exchange VMSCA is established, and the call routed to a gateway exchange GMSCA of the network. Second, GMSCA analyzes the MSISDN number MSISDN(B) of the called subscriber B, and contacts the gateway MSC GMSCB of B using the message IAM (Initial Address Message). In response to this message, GMSCB asks the home location register HLRB of subscriber B for the necessary routing information by sending the message SRI (Send Routing Information). Having received the message, HLRB checks the subscription information to find out the VLR address of the called subscriber and to see whether the call should be barred or transferred to a third party. If the call is to be connected to subscriber B, HLRB sends the current visitor location register VLRB of subscriber B the message PRN (Provide Roaming Number). In response VLRB returns a roaming number MSRN (Mobile Subscriber Roaming Number) identifying the called subscriber in this VLR. VLRB returns to HLRB the MSRN of subscriber B in message PRN ACK (PRN Acknowledgment), and HLRB forwards it to GMSCB in the message SRI ACK. Using the received identification MSRN and the message IAM(MSRN), GMSCB establishes a connection to the MSC VMSCB currently visited by subscriber B. To identify the called subscriber of the roaming number with his identity number IMSI VMSCB queries VLRB, and VLRB returns this information. According to the specifications, this is done using messages SIIC (Send Info for Incoming Call) and CC (Complete Call). Having the identity of the called subscriber, VMSCB now establishes the final leg of the call to the called subscriber using the message SETUP.
In GSM, the basic principle in charging is that the calling subscriber pays the leg between himself and the home network HPLMNB of the called subscriber. The rest of the route is paid by the called subscriber. The idea here is that the calling subscriber shall not have to pay a surprisingly high price for her call, e.g., when the called subscriber is roaming in a distant country. According to this principle, when the A subscriber in Finland calls a mobile subscriber whose home network is in New Zealand but who is currently roaming in Finland, subscriber A pays the leg from Finland to New Zealand and subscriber B the leg from New Zealand back to Finland.
ETSI (European Telecommunications Standards Institute) is specifying into the GSM system the method of optimal routing to be used as an alternative for the above described normal call routing. The basic idea of the optimal routing method, shown in FIG. 4, is to route the call using the most direct route between subscribers A and B. Instead of immediately establishing a connection to the gateway MSC GMSCB of the home network of the called subscriber, the gateway MSC GMSCA of the calling subscriber sends the home location register HLRB of the called subscriber the request SRI for routing information. HLRB asks the VLRB for a roaming number, and forwards the routing number to GMSCA. Using the roaming number, GMSCA establishes a connection directly to VMSCB visited by the called subscriber. Thus, the call is not necessarily routed through the home network of subscriber B, but the home network is only contacted in the HLR inquiry. Using optimal routing, the interexchange call expenses and thus the total call price can be reduced.
MoU (Memorandum of Understanding), an organization of the GSM operators, has specified two conditions to be met by the implementation of optimal routing:
1. using the optimal routing may not increase the call price for any subscriber (tariff requirement), and
2. at least in the first phase of the service, one call leg is always paid by one subscriber.
Because the expenses caused by different legs can not be assumed to be known in the first phase of optimal routing, the optimal routing is used in the first phase only if both GMSCA and VMSCB or both VMSCB and HLRB are in the same country. In this case, the price of the call for at least for one of the subscribers does not exceed the maximum of a national call to a mobile subscriber.
The aim of optimal routing is to route the call as directly as possible. Thus, the HLR inquiry giving information about the location of the called subscriber must be done as early as possible in the establishment procedure of the call. If the calling subscriber belongs to the same mobile telecommunications system as the called subscriber, the inquiry can be made by the gateway MSC GMSCA of subscriber A. If the calling subscriber does not belong to the same system, the call can be routed to a network element capable of doing HLR inquiries according a prespecified prefix number, for example.
Let us now study the establishment of an optimally routed call with reference to FIG. 5, which shows the signaling used in the establishment procedure. The call is first routed to an interrogating network IPLMN (Interrogating PLMN) capable of interrogating the HLR. VMSCA receives the request SETUP to establish a call specifying the MSISDN identity of the called subscriber. This request is forwarded to a gateway exchange GMSCA. If GMSCA supports optimal routing (OR1=Y, Yes), it sends the home location register HLRB of the called subscriber an HLR inquiry SRI(B, OR) identifying the called subscriber B and the request for optimal routing OR. If HLRB supports optimal routing (OR2=Y), it checks whether the tariff requirement for optimal routing is met. This requirement is that:
the directly routed call is cheaper than the leg between subscriber A and HPLMNB or
the directly routed call is cheaper than the leg between HPLMNB and subscriber B.
If at least one of these requirements is met (OR3=Y) and there are no other obstacles for establishing the call, HLRB sends the current VLR VLRB of the called subscriber a request PRN for a roaming number.
If VLRB supports optimal routing (OR4=Y), it returns the roaming number MSRN in the message PRN ACK, and HLRB relays it to GMSCB in the message SRI ACK. GMSCA then establishes a connection to VMSCB directly using the received MSRN in message IAM.
If at least one of the conditions OR2, OR3 or OR4 are not met, HLRB returns to GMSCA an error message SRI ERROR. When GMSCA receives this message or when it does not support optimal routing, it continues the call establishment using the normal routing procedure shown in FIG. 3.
The advantages of optimal routing are somewhat limited by the tariff requirements stipulating that optimal routing must not cause the call to be more expensive to any of the subscribers participating in the call. Let us consider a situation where the bills of both the calling and the called subscriber are paid by the same company. It is in the interest of the company to minimize the total cost of the call, i.e. the sum of the bills of subscribers A and B. Let us study a situation where A calls from France to B, whose home network is in Finland but who is roaming in China. The direct route from France to China is, nevertheless, more expensive than either of the individual legs, i.e. more expensive than the leg from France to Finland and more expensive than the leg from Finland to China. Therefore, to comply with the tariff requirements stipulating that the directly routed call may not become more expensive than the normally routed call to any subscriber, the normal routing is selected. However, for the company paying the bill, the optimal solution would be to route the call directly and thus minimize the total cost.
The objective of this invention is to solve the above-mentioned problem of the prior art. This objective is achieved using the method and apparatus specified in the independent claims.
The basic idea of this invention is to specify a condition for routing the call directly which is independent of the tariff requirement for direct routing.
According to one embodiment, the calls within a pre-specified user group are always routed directly. The invention can be implemented, e.g., by adding to the closed user group information of the subscriber a direct routing flag indicating that the calls within the closed user group are to be routed directly. The decision to route the call directly can then be made on the basis of the closed user group checks and the value of the direct routing flag of the calling subscriber.
According to another embodiment, the calling subscriber can specify that his calls are always to be routed directly. This information is added into the subscriber information preferably at the visitor location register of the subscriber.
In still another embodiment, the call is specify to be routed directly on a call per call basis. This can be achieved by adding to the call information a flag indicating that this call is to be routed directly.